U.S. patent application number 11/687907 was filed with the patent office on 2008-09-25 for method for manufacturing a workpiece by friction welding to reduce the occurrence of abnormal grain growth.
This patent application is currently assigned to The Boeing Company. Invention is credited to John A. Baumann, Richard J. Lederich.
Application Number | 20080230584 11/687907 |
Document ID | / |
Family ID | 39773696 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080230584 |
Kind Code |
A1 |
Lederich; Richard J. ; et
al. |
September 25, 2008 |
Method for Manufacturing a Workpiece by Friction Welding to Reduce
the Occurrence of Abnormal Grain Growth
Abstract
A method of manufacturing a workpiece is provided. The method
generally includes friction stir welding at least one structural
member, selectively removing material from the surfaces of the
workpiece at the location of a friction stir weld joint, and
thereafter subjecting the workpiece to a solution treat, quench,
and age treatment. By selectively removing regions from the
surfaces of the workpiece that are defined by nonuniform material
properties adapted to nucleate nonuniform grain growth during the
solution treat, quench, and age treatment, a subsequent grain
growth during the thermal treatment can be at least partially
prevented.
Inventors: |
Lederich; Richard J.; (Des
Peres, MO) ; Baumann; John A.; (St. Charles,
MO) |
Correspondence
Address: |
ALSTON & BIRD, LLP
BANK OF AMERICA PLAZA, 101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
The Boeing Company
|
Family ID: |
39773696 |
Appl. No.: |
11/687907 |
Filed: |
March 19, 2007 |
Current U.S.
Class: |
228/112.1 |
Current CPC
Class: |
B23K 20/1225 20130101;
C22F 1/04 20130101 |
Class at
Publication: |
228/112.1 |
International
Class: |
B23K 20/12 20060101
B23K020/12 |
Claims
1. A method of manufacturing a workpiece, the method comprising:
friction stir welding at least one structural member to form a
workpiece defining first and second surfaces and a friction stir
weld joint extending between the first and second surfaces, and
thereby forming a region near each of the first and second surfaces
defined by nonuniform material properties adapted to nucleate
nonuniform grain growth during a solution treat, quench, and age
treatment; selectively removing material from the first and second
surfaces of the workpiece at the location of the friction stir weld
joint and thereby removing the regions from each of the surfaces;
and thereafter subjecting the workpiece to a solution treat,
quench, and age treatment, wherein grain growth during the solution
treat, quench, and age treatment is at least partially prevented by
the removal of the regions from each surface.
2. A method according to claim 1 wherein said friction stir welding
step comprises friction stir welding first and second plates in an
abutting relationship such that the structural members
cooperatively define the first and second surfaces, each of the
surfaces having a planar configuration at the friction stir weld
joint.
3. A method according to claim 1 wherein said selectively removing
step comprises mechanically milling material from each of the first
and second surfaces.
4. A method according to claim 1 wherein said selectively removing
step comprises removing a thickness of material of between about
0.050 inch and 0.150 inch from each surface.
5. A method according to claim 1 wherein said selectively removing
step comprises removing a thickness of material of about 0.100 inch
from each surface.
6. A method according to claim 1 wherein said selectively removing
step comprises removing a width of material from each surface at
least as great as the width of a heat affected zone of the friction
stir weld joint.
7. A method according to claim 1 wherein said selectively removing
step comprises removing material having a grain size greater than a
material of the workpiece outside of the friction stir weld joint
such that the material at the friction stir weld joint after the
subjecting step is characterized by a grain size less than a
predetermined maximum grain size.
8. A method according to claim 1 wherein said selectively removing
step comprises removing material having a relatively greater
concentration of oxidized material relative to a remaining material
of the weld joint.
9. A method according to claim 1 wherein the predetermined maximum
grain size of the material at the friction stir weld joint is about
200 microns.
10. A method according to claim 1 wherein the predetermined maximum
grain size is about 10 times the grain size of the material of the
workpiece outside of the friction stir weld joint.
11. A method according to claim 1 wherein the predetermined maximum
grain size is about 20 times the grain size of the material of the
workpiece outside of a nugget of the friction stir weld joint.
12. A method according to claim 1 wherein said subjecting step
comprises heating the workpiece to a solution treat temperature to
perform a solution treatment, subsequently quenching the workpiece,
and subsequently aging the workpiece at an aging temperature less
than the solution treat temperature.
13. A method according to claim 1, further comprising providing the
at least one structural member formed of an aluminum alloy.
14. A method according to claim 1, further comprising solution
treating the at least one structural member prior to the friction
stir welding step.
15. A method according to claim 1, further comprising, prior to
said selectively removing step, determining a minimum thickness of
the material to be selectively removed from each of the first and
second surfaces of the workpiece in said removing step to thereby
substantially prevent a nonuniform grain growth during the solution
treat, quench, and age treatment.
16. A method according to claim 1 wherein said determining step
comprises providing at least two test members, each test member
defining a friction stir weld joint, removing different thicknesses
of material from surfaces of the test members at locations of the
friction stir weld joints, subjecting the test members to thermal
treatments, and testing the test members to determine the minimum
thickness of the material to be selectively removed.
17. A method according to claim 1 wherein said friction stir
welding step comprises friction stir welding first and second
plates in an abutting relationship such that the structural members
cooperatively define the first and second surfaces, each of the
surfaces having a substantially planar configuration at the
friction stir weld joint.
18. A workpiece manufactured according to the method of claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1) Field of the Invention
[0002] Embodiments of this invention relate to the manufacture of a
workpiece and, more specifically, to a method of manufacture in
which a workpiece is friction welded and the occurrence of abnormal
grain growth in the workpiece during a heat treatment operation is
reduced or prevented.
[0003] 2) Description of Related Art
[0004] Friction stir welding is a process in which a rotating tool,
such as a pin or probe, is urged into and/or through a workpiece,
e.g., to join multiple structural members of the workpiece in a
solid state or to repair cracks or other defects in a workpiece.
According to one conventional friction stir welding process, two
structural members are disposed in an abutting or overlapping
configuration to define an interface therebetween. A shoulder of
the tool that can be flat, concave, or otherwise contoured is urged
against one side of the structural members so that a pin of the
tool that extends from a shoulder is plunged into the two
structural members. The pin is then translated through the
structural members along the interface. The motion of the rotating
tool generates frictional heating, thereby forming a region of
plasticized material in the structural members that is mixed
plastically by the tool. Upon cooling of the plasticized material,
the members of the workpiece are joined along the weld joint.
Friction stir welding is further described in U.S. Pat. No.
6,994,242 to Fuller, et al. and U.S. Pat. No. 5,460,317 to Thomas
et al., the entire contents of which are incorporated herein by
reference.
[0005] Friction stir welding has been demonstrated to be a
successful joining method and is used for a variety of materials.
However, in some cases the friction welding operation leads to a
change in the material properties proximate the weld joint. In
particular, the friction stir welding process can result in changes
in the material, and these material changes can affect the
performance of the material in use or during subsequent processing
operations. For example, in a typical operation of friction stir
welding high strength aluminum alloys, such as 7000 series alloys,
the heat associated with friction stir welding typically results in
coarsening of precipitates in a heat affected zone near the
friction stir welding joint, thereby decreasing the strength of the
material in the heat affected zone. Such high strength aluminum
alloys can be subjected to a heat treatment process after welding
to restore the strength of the material in the heat affected zone
to a condition similar to that of the parent material. Although
such a heat treatment process can be effective for improving the
properties of the material in the heat affected zone, the heat
treatment can also nucleate abnormal grain growth in the nugget
portion of the weld joint. That is, the grain size of the material
in the weld joint can grow nonuniformly and undesirably during the
heat treatment. As a result, when the resulting joint is subjected
to loading, the nugget typically deforms nonuniformly, such that
the ductility of the joint is reduced (i.e., the joint is more
brittle) and the joint is capable of less elongation than the
unwelded parent material. Reductions in the ductility of the
material can be an indication of reduced fatigue performance. The
degree of abnormal grain growth and the amount of strength
reduction can be affected by such factors as the heat generated
during the friction stir welding operation, the thickness of the
joint or other factors that affect the amount of heat generated
during friction stir welding, the original material properties, and
the characteristics of the heat treatment operation. It is
appreciated that other mechanisms impacting this grain growth may
be at work. In some cases, the strength reduction can be
significant. For example, in a 1-inch thick friction stir weld
joint between members of 7000 series aluminum alloys, the strength
at the friction stir weld joint can be reduced about 30% relative
to the parent material. The use of relatively low-temperature
post-weld aging instead of conventional thermal heat treatments has
not been found to effectively achieve high strengths at the weld
joints.
[0006] Thus, a need exists for an improved manufacturing method for
friction welding in which the occurrence of abnormal grain growth
and strength reduction can be reduced or prevented.
SUMMARY OF THE INVENTION
[0007] Embodiments of the present invention provide methods of
manufacturing a workpiece by friction stir welding and an
associated workpiece, in which material is selectively removed from
the surfaces of the workpiece at the location of a friction stir
weld joint to at least partially prevent grain growth during a
subsequent thermal treatment.
[0008] According to one embodiment, the method includes friction
stir welding at least one structural member to form a workpiece
defining first and second surfaces and a friction stir weld joint
extending between the first and second surfaces, and thereby
forming regions near the first and second surfaces defined by
nonuniform material properties adapted to nucleate nonuniform grain
growth during a solution treat, quench, and age treatment. The
workpiece can include first and second plates that are provided in
an abutting relationship so that the structural members
cooperatively define the first and second surfaces, each of the
surfaces having a substantially planar configuration at the
friction stir weld joint. Material is selectively removed from the
first and second surfaces of the workpiece at the location of the
friction stir weld joint, e.g., by machining, to thereby remove the
regions from each of the surfaces. Thereafter, the workpiece is
subjected to a solution treat, quench, and age treatment, wherein a
grain growth during the solution treat, quench, and age treatment
is at least partially prevented by the removal of the regions from
each surface.
[0009] In some cases, a thickness of material of between about
0.050 inch and 0.150 inch, such as a thickness of about 0.100, can
be removed from each surface. Material can be removed from each
surface in a width that is at least as great as the width of a
mechanically affected zone of the friction stir weld joint, and
typically at least as great as the width of a heat affected zone of
the friction stir weld joint. Material having nonuniformities, such
as a relatively greater concentration of oxidized material relative
to a remaining material of the weld joint or a grain size greater
than a material of the workpiece outside of the friction stir weld
joint, can be removed so that the material at the friction stir
weld joint after the thermal treatment is characterized by a grain
size less than a predetermined maximum grain size, and the
predetermined maximum grain size can be about 200 microns, about 10
times the grain size of the material of the workpiece outside of
the friction stir weld joint, and/or about 20 times the grain size
of the material of the workpiece outside of a nugget of the
friction stir weld joint. The at least one structural member can be
formed of an aluminum alloy, such as 7050-T7451, and can be
solution treated prior to friction stir welding.
[0010] According to one embodiment, the method further includes,
prior to selectively removing the material, determining a minimum
thickness of the material to be selectively removed from each of
the first and second surfaces of the workpiece in the removing
operation to thereby substantially prevent a nonuniform grain
growth during the solution treat, quench, and age treatment. For
example, two or more test members can be provided, each test member
defining a friction stir weld joint, and different thicknesses of
material can be removed from surfaces of the test members at
locations of the friction stir weld joints, before subjecting the
test members to thermal treatments and then testing the test
members to determine the minimum thickness of the material to be
selectively removed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing and other advantages and features of the
invention, and the manner in which the same are accomplished, will
become more readily apparent upon consideration of the following
detailed description of the invention taken in conjunction with the
accompanying drawings, which illustrate preferred and exemplary
embodiments, but which are not necessarily drawn to scale,
wherein:
[0012] FIG. 1 is a perspective view illustrating a conventional
friction stir welding apparatus configured to form a friction stir
weld butt joint in a workpiece that includes two abutting
structural members;
[0013] FIG. 2 is a section view illustrating a conventional
friction stir weld joint formed with the welding apparatus of FIG.
1;
[0014] FIG. 3 is a section view illustrating the friction stir weld
joint of FIG. 2 after a conventional heat treatment;
[0015] FIGS. 4-6 are enlarged section views illustrating portions
of the friction stir welding joint of FIG. 3;
[0016] FIG. 7 is perspective view schematically illustrating a
friction stir weld joint formed according to one embodiment of the
present invention after removal of first and second regions and
before a thermal heat treatment operation;
[0017] FIG. 8 is a section view illustrating a friction stir weld
joint formed according to one embodiment of the present invention
after a thermal heat treatment operation;
[0018] FIG. 9 is a section view illustrating a friction stir weld
joint formed according to another embodiment of the present
invention;
[0019] FIG. 10 is a perspective view illustrating a test member
configured for testing according to one embodiment of the present
invention;
[0020] FIG. 11 is a perspective view partially illustrating five
test members formed according one embodiment of the present
invention after tensile testing thereof,
[0021] FIG. 12 is an enlarged perspective view illustrating one of
the partial test members of FIG. 11; and
[0022] FIG. 13 is a flow chart illustrating the operations for
manufacturing a workpiece according to one embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
[0024] Referring now to the drawings and, in particular, to FIG. 1,
there is shown a conventional friction stir welding apparatus 10
for friction stir welding a workpiece 12. The friction stir welding
apparatus 10 includes a friction stir welding tool having a pin 14
that extends from a shoulder 16, and at least one actuator 18 for
rotating the tool and moving the tool through the workpiece 12 to
form a friction weld joint 20. For example, the friction stir
welding tool can be engaged to a chuck, spindle, or other member
that is engaged to the actuator 16. The actuator 16 can be any of
various types of actuating devices, including electric, hydraulic,
or pneumatic devices, any of which can include a mechanical
linkage. For example, the actuator 16 can be part of a machine,
such as a milling machine or a drill, which is structured for
rotating the friction stir welding tool in a direction 22 and
translating the tool through the workpiece 12 in a direction 24 of
the workpiece 12. The actuator 16 be operated manually, but
preferably is operated by a computer, microprocessor,
microcontroller, or other controller 26, which can be programmed to
operate according to a schedule such as a schedule stored in or
created by a computer software program.
[0025] The term "workpiece" is not meant to be limiting, and it is
understood that the workpieces 12 that are friction welded
according to the present invention can include one or more
structural members, which can be configured in various
configurations. For example, as shown in FIG. 1, two structural
members 28, 30 are positioned so that the edges of the members 28,
30 are in abutting contact to define an interface 32 therebetween
that can be welded to form the joint 20, e.g., a butt weld joint as
shown in FIG. 1. Alternatively, the apparatus 10 can be used to
form other types of joints such as a lap joint that is formed by
overlapping faying surfaces of the structural members and welding
through an interface of the faying surfaces to form a lap joint
that extends along the interface. The structural members can also
be positioned and welded in other configurations, and any number of
structural members can be joined by the joint. In another
embodiment, the workpiece can include a single structural member,
and the friction stir welding apparatus 10 can be used to form a
weld joint in the member, e.g., to repair a crack, hole, or other
defect therein or to affect the material properties of the
structural member. In some cases, the workpiece 12 can be further
processed after friction stir welding, such as by machining the
workpiece 12 to a desired size or configuration. Methods for
friction stir welding a preform that is subsequently trimmed by
machining are discussed in U.S. Pat. No. 7,156,276, the entire
content of which is incorporated herein by reference.
[0026] As illustrated in FIG. 1, the friction stir welding tool
includes the shoulder 16 and the pin 14 extending therefrom. The
pin 14 and shoulder 16 are preferably formed of a material having
high strength and heat resistance. The shoulder 16 is structured to
be urged against the workpiece 12 such that the pin 14 is inserted
into the workpiece 12, e.g., into the interface as shown in FIG. 1.
Alternatively, the tool can include first and second shoulders that
are structured in an opposed configuration with a pin extending
between the shoulders such that the shoulders can be disposed
opposite the workpiece 12 and frictionally engaged to the opposite
surfaces of the workpiece 12 therebetween. In either case, each
shoulder of the tool can define a surface that is generally flat,
tapered, concave, convex, or otherwise shaped, e.g., to engage the
workpiece 12 and prevent "plowing," in which plasticized material
from the workpiece 12 is pushed radially outside the circumference
of the shoulder as the tool is moved along the workpiece 12.
Further, each shoulder can define one or more frictional features,
e.g., raised portions or surfaces such as threads, bumps, or ribs
that are structured to frictionally engage the workpiece 12. For
example, a spiral thread can be provided on each shoulder to engage
the workpiece 12. The pin defines a stirring portion that engages
the workpiece 12 during welding, and the stirring portion of the
pin can be cylindrical or can define a variety of shapes and
contours including helical threads, circumferential grooves,
ridges, tapers, steps, and the like.
[0027] FIG. 2 illustrates a cross-section of the friction stir
welding joint 20 formed in the workpiece 12 by the conventional
friction stir welding process of FIG. 1. In the illustrated
embodiment, each of the structural members 28, 30 of the workpiece
12 is a plate formed of 7050 aluminum alloy having a thickness of
1.3 inch and an initial (i.e., pre-weld) average grain size of
between about 20 and 25 microns. The weld joint 20 defines a weld
nugget 34 and a heat affected zone 36. The nugget 34 includes a
thermal mechanical zone, i.e., a region where the material has been
plasticized and mixed by the action of the friction stir welding
pin 14. The heat affected zone 36 outside of the nugget 34 is
generally defined by material that is not plasticized or mixed
during welding, but which is affected by the high temperatures
associated with the friction stir welding operation.
[0028] The friction stir weld joint 20 is characterized by
nonuniform material properties, i.e., nonuniformities, that affect
the joint 20 throughout further processing and use. The amount and
degree of such nonuniformities typically varies throughout the
joint 20. While the nonuniformities are not easily identified, it
is believed that the distribution of such nonuniformities is
greatest in portions of the joint 20 near the opposite surfaces 38,
40 of the workpiece 12. While the present invention is not limited
to any particular theory of operation, it is believed that
nonuniformities are formed near, i.e., proximate, the surfaces 38,
40 as a result of the mixing of surface oxides from the surfaces
into the joint 20; as a result of excessive strains; and/or as a
result of other factors that affect the material of the joint 20
more at the surfaces of the workpiece 12 than the center of the
joint 20. It is further believed that the nonuniformities present
in the weld joint 20 after friction stir welding can nucleate
abnormal grain growth during subsequent treatments. That is, each
nonuniformity in the joint 20 is adapted to nucleate, or stimulate,
the growth of grains in the local region of the nonuniformity
during subsequent processing to sizes that are larger than a normal
grain size of the joint 20, such as the average grain size in a
central portion of the joint 20 that is substantially unaffected by
the nonuniformities. In some cases, the abnormal grain growth
during a heat treatment process can result in grain sizes that
greatly exceed the grain size of the parent material of the
workpiece 12 and the material in the central portion of the joint
20. For example, in some cases, grains can grow to sizes greater
than 10 times the grain size of the parent material of the
workpiece 12, i.e., the material of the workpiece 12 outside the
friction stir weld joint 20, or to sizes greater than 20 times the
grain size of the material in the nugget 34 of the weld joint
20.
[0029] FIG. 3 illustrates a cross-section of the friction stir
welding joint 20 of FIG. 2 after a conventional post-weld heat
treatment process that includes heating the workpiece 12 to a
temperature of 890.degree. F. and maintaining this temperature for
an hour, quenching the workpiece 12 in a relatively cool liquid,
and aging the workpiece 12, e.g., at a temperature of 250.degree.
F. for 6 hours and then a temperature of 325.degree. F. for 24
hours. Such a heat treatment process is typically referred to as a
solution treat, quench, and age process, or STQA process.
[0030] As illustrated, the weld joint 20 defines various portions
having different material properties and, in particular, different
grain structures. For example, as illustrated in FIG. 4, a central
portion 42 of the joint 20 is characterized by a substantially
uniform material with a substantially uniformly refined grain
structure. The grain structure in the central portion 42 of the
joint 20 is refined relative to the initial material of the
workpiece 12, e.g., with average grain sizes of less than 20
microns, such as an average grain size of between about 2 and 15
microns.
[0031] Portions of the joint 20 proximate the surfaces 38, 40 of
the workpiece 12 are typically characterized by less uniformity and
greater grain sizes than the central portion 42 of the joint 20. In
particular, as illustrated in FIG. 5, at a first portion 44
adjacent the first surface 38 of the workpiece 12, the grain size
of the material is substantially greater than the central portion
42. More particularly, the average grain size in the first portion
44 is about 400 microns. It is believed that the large grains in
the first portion 44 are formed during the STQA process as a result
of grain growth nucleated or stimulated by the existence of surface
oxides from the first surface 38 of the workpiece 12 before
friction welding that were mixed into the joint 20 during welding
by the pin 14 and/or the shoulder 16. Although such surface oxides
can be redistributed throughout the entire height of the weld joint
20, the distribution of the surface oxides is typically greater in
the first portion 44 of the joint 20, i.e., nearest the first
surface 38 of the workpiece 12.
[0032] As illustrated in FIG. 6, a second portion 46 adjacent the
second surface 40 of the workpiece 12 is also characterized by less
uniformity and greater grain sizes than the central portion of the
joint 20. In particular, the material in the second portion 46 has
a grain size that is substantially greater than the central portion
42, e.g., about 200 microns. It is believed that the large grains
in the second portion 46 are formed during the STQA process as a
result of grain growth nucleated or stimulated by the existence of
predispositioned material that is formed in or near the second
portion 46 during friction welding, i.e., material that is
predispositioned as a result of the friction welding process to
nucleate grain growth that is abnormally large relative to the
other material. Such predispositioning of the material may possibly
be a result of high mechanical strains that are imparted to the
material during the friction welding process.
[0033] It is believed that the removal of material having
nonuniformities from the workpiece 12 before the thermal treatment
can reduce or eliminate the subsequent abnormal growth of grains in
the first and second portions 44, 46 of the workpiece 12. That is,
by removing nonuniformities in the material of the weld joint 20,
such as oxides that are mixed into the joint 20 or predispositioned
or highly sensitized material, the subsequent nucleation of
abnormal grain growth during the heat treatment process can be
prevented or reduced, thereby at least partially preventing the
grain growth that would otherwise occur during the heat treatment
process. Thus, according to one method of the present invention,
the material removed from one or both of the surfaces 38, 40 of the
workpiece 12 can have relatively greater concentrations of oxidized
material, highly strained material, or other nonuniformities
relative to the remaining material of the weld joint 20.
[0034] The nonuniformities can be removed by selectively removing
material from the first and second surfaces 38, 40 of the workpiece
12 at the location of the friction stir welding joint 20. Further,
the amount of material that is selectively removed can be
significantly less than the amount of material that would otherwise
be affected by the abnormal grain growth. In other words, the
removal of a relatively small region of material can reduce or
prevent the abnormal grain growth throughout a larger portion of
the joint 20 that includes material not removed from the joint
20.
[0035] In some cases, the amount of material removed from the joint
20 can be a relatively thin layer from each surface 38, 40. For
example, FIG. 7 schematically illustrates the removal of regions
from the first and second surfaces 38, 40 of a workpiece 12
according to one embodiment of the present invention. A first
region, indicated by reference numeral 48, has been removed from
the first surface of the workpiece 12, and a second region,
indicated by reference numeral 50, has been removed from the second
surface of the workpiece 12. The thickness t.sub.1, t.sub.2 of each
removed region 48, 50 is exaggerated in FIG. 7 for purposes of
illustrative clarity. In one typical embodiment, the thickness
t.sub.1, t.sub.2 of each region 48, 50 removed from each surface
38, 40 is equal to or less than about 0.200 inch, such as between
about 0.050 inch and 0.150 inch, and in one specific embodiment,
about 0.100 inch.
[0036] The first and second regions 48, 50 are typically removed
mechanically. For example, a conventional computer numeric control
(CNC) machine or similar device can be used to move a rotating
machine tool over each surface 38, 40 and thereby mechanically
machine or mill the material from the surfaces 38, 40. In some
cases, the material can be removed with a machining tool that is
actuated by the same machine used for forming the friction stir
weld joint. The material can be removed from the entire surfaces
38, 40 so that the contour of each surface 38, 40 is substantially
the same after removal of the material. For example, if the
surfaces 38, 40 are initially planar, or substantially planar, a
uniform thickness of material can be removed across the entire area
of each surface 38, 40 so that each surface 38, 40 is also planar
after the removal operation. Alternatively, the material can be
removed from an area that is smaller than the entire surfaces 38,
40, and typically is removed only from an area proximate the weld
joint 20. In particular, the material can be removed from each
surface 38, 40 only at locations coincident with the weld joint 20.
As shown in FIG. 7, the material is removed from each surface 38,
40 at the location of the friction stir weld joint 20 and, more
particularly, from the nugget 34 and the heat affected zone 36 of
the joint 20. The sizes and/or configurations of the regions 48, 50
removed from the first and second surfaces 38, 40 can be different,
e.g., to correspond to the different sizes of the heat affected
zone 36 at each surface 38, 40. The area of removal is typically
slightly larger than the heat affected zone 36 as shown in FIG. 7.
In some cases, a local portion of the surfaces 38, 40 may be
slightly curved and, hence, substantially planar, even though the
workpiece 12 defines a nonplanar configuration overall. For
example, in the case of a workpiece 12 that defines a relatively
large cylindrical shape, such as a cylinder having a diameter of 20
feet or more, the curvature of the inner and outer surfaces is
substantially planar (i.e., only slightly curved) even though the
workpiece 12 defines a cylindrical shape.
[0037] After the removal of the first and second regions 48, 50,
the workpiece 12 is subjected to a thermal heat treatment
operation, such as a STQA treatment, during which grain growth in
the workpiece 12 is at least partially prevented by the removal of
the first and second regions 48, 50. For example, the STQA
treatment can include heating the workpiece 12 to a solution treat
temperature to perform a solution treatment, subsequently quenching
the workpiece 12, and subsequently aging the workpiece 12 at an
aging temperature less than the solution treat temperature. In one
typical embodiment, the workpiece 12 is heated to a solution treat
temperature of about 890.degree. F. and maintained at this
temperature for about an hour. Thereafter, the workpiece 12 is
quenched in a relatively cool liquid, and then aged at a
temperature of about 250.degree. F. for about 6 hours and then a
temperature of about 325.degree. F. for about 24 hours.
[0038] FIG. 8 illustrates a workpiece 12 formed according to one
embodiment of the present invention, after a thermal heat treatment
operation has been performed. In this embodiment, a thickness
t.sub.1, t.sub.2 of about 0.100 inch was removed from each of the
surfaces 38, 40 of the workpiece 12 before a STQA treatment as
described above. FIG. 9 illustrates a similar workpiece 12 formed
according to another embodiment, in which a thickness t.sub.1,
t.sub.2 of about 0.150 inch was removed from each of the surfaces
38, 40, and the workpiece 12 was then subjected to the same STQA
treatment. As illustrated in FIGS. 8 and 9, the workpieces 12 are
not characterized by any (or any substantial) abnormal grain
growth. In other words, by removing the nonuniformities at the
first and second surfaces 38, 40 of the workpiece 12 prior to the
STQA treatment, the undesirable grain growth that would otherwise
have occurred during the STQA treatment was prevented. Instead, the
friction stir weld joints 20 in the workpieces 12 of FIGS. 8 and 9
have uniform grain structures throughout, including the first and
second portions 44, 46 of each joint 20 as well as the central
portion 42 of each joint 20. It should be noted that the removal of
material prevents abnormal grain growth in other material that is
not removed. In other words, by removing a relatively small amount
of material prior to the STQA treatment, undesirable grain growth
is reduced or prevented throughout the joint 20, including the
portions 44, 46 of the joint 20 adjacent the surfaces 38, 40 of the
workpiece 12 where abnormal grain grown would otherwise have
occurred.
[0039] The thickness of material that is to be removed from each
surface 38, 40 can be determined before the removal operation and,
in some cases, before the workpiece 12 is friction stir welded. For
example, prior to friction stir welding a workpiece 12, one or more
test coupons or test members can be prepared for determining the
minimum thickness that must be removed to substantially prevent the
abnormal grain growth during a particular heat treatment operation.
A test coupon or test member is typically a small member that has
material properties similar to those of the workpiece 12 and which
can be tested prior to manufacture of the workpiece 12, e.g., using
a standard tensile test device that applies a tensile force to the
member until failure. A test member 52 is schematically illustrated
in FIG. 10. As illustrated, the test member 52 defines a grip
portion 54, 56 at each end and a test portion 58 therebetween. The
test portion 58 has a cross-sectional size of about 1.3 inches by
about 0.25 inches. The test portion 58 is friction stir welded
using a friction stir welding pin having a length of about 1.255
inches, such that, when a shoulder of the friction stir welding
tool is urged against a first surface 60 of the test portion 58,
the friction stir welding pin extends through the first surface 60
and nearly to an opposite second surface 62 of the test member 58.
The rotating friction stir welding pin is translated through the
test portion 58 in direction 64, thereby forming a friction stir
weld joint 20 in the test member 52. Thereafter, the test member 52
can be milled or machined and then subjected to a STQA treatment.
The finished test member 52 can then be tested, e.g., using a
conventional tensile test device to grip the grip portions 54, 56
of the test member 52 and apply a tensile force in directions 66
until the test portion 58 breaks.
[0040] FIG. 11 partially illustrates five test members 52,
individually denoted 2-14-00, 2-14-50, 2-14-100, 2-14-150, and
2-14-200, that were manufactured and tested prior to the
manufacture of the workpieces 12 illustrated in FIGS. 8 and 9. Each
of the test members 52 defines a friction stir weld joint 20 formed
as described above, and the test members 52 were subjected to the
STQA treatment described above. Before the STQA treatment, various
thicknesses of material were removed from the different test
members 52. In particular, a thickness of between 0 inch and 0.200
inch was removed by machining from each surface 60, 62. The test
members 52 were then subjected to the tensile test as described
above to determine the elongation and strength of each test member
52 before breaking the test member 52 at the weld joint 20. FIG. 11
illustrates one side of each test member 52 after tensile failure
thereof so that a break surface 68 of each member 52 is visible.
Each test member 52 is identified by a "Spec ID" shown in FIG. 11
and the table below. The final portion of each Spec ID indicates
the thickness (in thousandths of an inch) of the material machined
from each surface 60, 62 of the test member 52 at the location of
the friction stir weld joint 20 prior to the STQA treatment and the
tensile test. That is, no material was removed from the first test
member 52, denoted 2-14-00; a thickness of 0.050 inch was removed
from each surface 60, 62 of the second test member 52, 2-14-50; a
thickness of 0.100 inch was removed from each surface 60, 62 of the
third test member 52, 2-14-100; a thickness of 0.150 inch was
removed from each surface 60, 62 of the fourth test member 52,
2-14-150; and a thickness of 0.200 inch was removed from each
surface 60, 62 of the fifth test member 52, 2-14-200.
TABLE-US-00001 Spec ID Fty (ksi) Ftu (ksi) % Elongation 2-14-00
66.2 69.9 2.6 2-14-50 66.0 72.4 4.7 2-14-100 65.7 72.5 5.7 2-14-150
65.8 72.2 4.7 2-14-200 66.7 73.1 5.2
[0041] The above table indicates the tensile yield strength (Fty),
the ultimate tensile strength (Ftu), and the percent tensile
elongation for each test member 52. The test members 52 of FIG. 11,
as well as the workpieces 12 of FIGS. 8 and 9, are formed of
aluminum alloy 7050-T7451, a conventional solution treated aluminum
alloy. For comparison, this aluminum alloy (outside of the portion
that is affected by the friction stir welding) is characterized by
the following properties: tensile yield strength (Fty) of 67 ksi,
ultimate tensile strength (Ftu) of 76 ksi, and tensile elongation
of 10 percent.
[0042] As evident from the above table, the first test member 52,
2-14-00, which was not machined before the STQA treatment,
fractured in a brittle manner, with little tensile elongation
(2.6%). The other test members 52, denoted 2-14-50, 2-14-100,
2-14-150, and 2-14-200, which were machined prior to the STQA
treatment, exhibited more ductile failures with greater elongation
(4.7% or greater). Further, the test members 52 that were machined
prior to the STQA treatment exhibited greater strengths that are
closer to that of the parent material (7050-T7451). As illustrated
in FIG. 12, a small amount of grain growth occurred near the second
surface 62 of the fourth test member 52, denoted 2-14-150; however,
the amount of grain growth was significantly less than in the
unmachined workpiece 12 illustrated in FIG. 3. Significant
improvements were achieved by the removal of 0.050 inch or more
from each surface 60, 62 and, in particular, by the removal of
0.100 inch or more from each surface 60, 62. In fact, the removal
of 0.100 inch from each surface 60, 62 achieved improved properties
similar to those of the test members 52 denoted 2-14-150 and
2-14-200, from which greater amounts of material were removed.
[0043] In other embodiments of the present invention, the
workpieces 12 can be manufactured without the use of such test
members 52, and the minimum amount of material to be removed from
each surface 38, 40 of a workpiece 12 can be determined in other
manners. For example, in some cases, the amount of material to be
removed can be determined according to testing or examination of
the workpiece 12 prior to thermal treatment. Alternatively, the
amount of material to be removed from each surface 38, 40 can be
determined by theoretical or analytical methods, or by other
empirical methods such as according to data determined from other
workpieces 12 of similar or dissimilar materials.
[0044] The removal of material from the surfaces 38, 40 generally
requires additional processing. Further, removal of significant
amounts of material from the workpiece 12 can increase the time and
expense for manufacture, as well as require the use of larger
workpiece 12 thicknesses in order to achieve the desired sizes
after machining. Accordingly, the minimum amount of material to be
removed from each surface 38, 40 generally refers to the minimum
thickness that must be removed from each surface 38, 40 so that no
or minimal abnormal grain growth occurs during the subsequent
thermal processing. The minimum thickness to be removed can differ
for the different surfaces 38, 40. That is, in some cases, it may
be necessary to remove a greater amount of material from one of the
surfaces 38, 40 than the other surface 38, 40 to reduce or prevent
abnormal grain growth during the thermal treatment.
[0045] It is appreciated that the thickness t.sub.1, t.sub.2 of
material that must be removed from each surface 38, 40 in order to
reduce and/or prevent abnormal grain growth during a subsequent
thermal treatment can vary depending on such factors as the
material of the workpiece 12, the dimensions and configuration of
the workpiece 12, the temperature that results in the workpiece 12
during friction stir welding, the type and size of friction stir
welding tools that are used, the operating parameters of the
friction stir welding machine such as the rotational and
translational speed of the friction stir welding tool, and the
like.
[0046] FIG. 13 illustrates the operations for manufacturing a
workpiece 12 according to one embodiment of the present invention.
In some cases, the method includes determining a minimum thickness
of material that is to be selectively removed. See block 100.
Before or after the minimum thickness is determined, at least one
structural member is provided. See block 102. In some cases, the at
least one structural member is solution treated or provided in a
solution treated condition, e.g., as aluminum alloy 7050-T7451. See
block 104. The at least one structural member is friction stir
welded to form a workpiece 12. See block 106. As a result of the
friction stir welding operation, regions are formed near the first
and second surfaces defined by nonuniform material properties
adapted to nucleate nonuniform grain growth during a subsequent
heat treatment. Material is then selectively removed from the first
and second surfaces of the workpiece 12 to thereby remove the
regions of nonuniform material properties. See block 108.
Thereafter, the workpiece 12 is subjected to a solution treat,
quench, and age treatment, with a grain growth during the solution
treat, quench, and age treatment being at least partially prevented
by the removal of the regions on each surface. See block 110.
[0047] The methods provided by the present invention can be used
for joining thick workpieces, such as plates or other members
having thicknesses of 1 inch or more. In some cases, the workpieces
can be subjected to significant strains, heat, and oxide
introduction during friction stir welding. In this regard, relative
to thin workpieces, such thick workpieces generally require larger
friction stir welding pins and slower speeds of translation during
friction stir welding, such that greater amounts of thermal energy
are generated during friction welding, potentially introducing
great nonuniformities in the resulting weld joints.
[0048] Friction stir welding can be used to process structural
members that are formed of a variety of materials including, but
not limited to, aluminum, aluminum alloys, titanium, titanium
alloys, steel, and the like. Non-metal materials can also be welded
with the friction stir welding apparatus, e.g., materials such as
polymers and the like. Further, the workpiece can include members
of similar or dissimilar materials, for example, structural members
formed of different metals, including metals that are unweldable or
uneconomical to join by conventional fusion welding techniques.
Unweldable materials, when joined by conventional fusion welding
techniques, produce relatively weak weld joints that tend to crack
during weld solidification. Such materials include aluminum and
some aluminum alloys, particularly AA series 2000 and 7000 alloys.
The use of friction stir welding permits workpieces formed of
unweldable materials to be securely joined. Friction stir welding
also can be used to securely join weldable materials to other
weldable and to unweldable materials.
[0049] The workpieces formed according to the present invention can
be used in a variety of applications, including, for example,
frames, panels, skins, airfoils, and the like for aeronautical and
aerospace structures such as aircraft and spacecraft, for marine
vehicles, automobiles, trucks and trailers, railcars, and the like,
as well as for other applications outside of the transportation
industry. The friction stir welded workpieces can be large and/or
can have curvilinear or other complex geometries. In some
applications, the structural members of the workpiece are joined in
geometrical configurations that make difficult, or prevent, access
to the opposing sides of the workpiece. For example, the structural
members can be joined to form a partially or fully closed body such
as a tube or an airplane wing. While friction stir welded butt
joints are illustrated in the application, it is appreciated that
the present invention can also be applied to the formation of other
joints, such as friction stir welded lap joints in which one member
overlaps another member.
[0050] Many modifications and other embodiments of the invention
will come to mind to one skilled in the art to which this invention
pertains having the benefit of the teachings presented in the
foregoing descriptions and the associated drawings. For example,
the structural members and/or the workpieces can be otherwise
processed before and/or after joining by friction welding. Such
processing can include cleaning the joining surfaces of the
structural members to remove oxidation or surface defects.
Therefore, it is to be understood that the invention is not to be
limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included
within the scope of the appended claims. Although specific terms
are employed herein, they are used in a generic and descriptive
sense only and not for purposes of limitation.
* * * * *